1. Field of the Invention
The invention under consideration relates to a heat-generating element of a heating device for heating air, comprising at least one PTC element and, lying on opposing side surfaces of the PTC element, electric strip conductors. Such a heat-generating element is known, for example, from EP 1 061 776, which is traced back to the current applicant.
In particular, the heat-generating element is deployed in an auxiliary heater for a motor vehicle, and comprises multiple PTC elements, arranged in a row, one behind the other, that are energized via electric strip conductors that extend parallel to one another and that lie flat on opposing sides of the PTC elements. The strip conductors are normally formed by parallel strips of metal. The heat-generating elements formed in this way are deployed in a heating device for heating air in a motor vehicle, where said heating device comprises multiple layers of heat-generating elements having heat-emitting elements that lie on their opposite sides. These heat-emitting elements are positioned so that they lie against the heat-generating elements in a relatively good heat-transferring contact by means of a holding device.
2. Description of the Related Art
With the aforementioned state of the art, a holding device of the heating device is formed by a frame in which multiple layers of heat-generating and heat-emitting elements that run parallel to one another are held by means of a spring bias. In an alternative development, which likewise discloses a generic heat-generating element and a generic heating device and that is described, for example, in EP 1 467 599, the heat-generating element is formed by multiple PTC elements arranged one behind the other, in a row in one level, said PTC elements also being called ceramic elements or positive temperature coefficient thermistors, and being energized on opposing side surfaces by strip conductors that lie on these side surfaces. One of the strip conductors is formed by a circumferentially closed profile, and the other strip conductor by a strip of metal that is supported at the circumferentially closed metal profile with an electrically insulating layer in between. The heat-emitting elements are formed by segments arranged in multiple parallel layers, said segments extending at right-angles to the circumferentially closed metal profile. In the generic heating device known from EP 1 467 599, multiple circumferentially closed metal profiles formed in the manner described in the preceding are provided, said metal profiles being arranged parallel to one another. To some extent, the segments extend between the circumferentially closed profiles and project beyond them to some extent.
In the case of the aforementioned heat-generating elements, there is a requirement that the electric strip conductors must be in good electrical contact with the PTC elements. Otherwise, the problem that arises is an increased transition resistance, which, particularly in the case of the use of heat-generating elements in auxiliary heaters for motor vehicles, can lead to local overheating due to the high currents. As a result of this thermal event, the heat-generating element can be damaged. Furthermore, the PTC elements are self-regulating resistance heaters that emit a lower heat output at an increased temperature, so that local overheating can lead to a disturbance in the self-regulating characteristics of the PTC elements.
In addition, at the high temperatures in the area of an auxiliary heater, vapours or gases can develop that can result in a direct hazard for persons in the passenger compartment.
Correspondingly problematic is also the use of generic heat-generating elements at high operating voltages, such as voltages up to 500 V, for example. For one thing, a problem here is that the air that flows against the heat-emitting elements carries moisture and/or dirt with it, which can penetrate into the heating device and cause an electric flashover, i.e., a short-circuit, here. On the other hand, there is fundamentally the problem of protecting persons working in the area of the heating device from the current-carrying parts of the heating device or of the heat-generating element.
In the case of heat-generating elements of the generic type, the PTC elements are usually arranged in a positioning frame that extends as a flat component essentially in the level of the PTC elements. The positioning frame serves the accurate positioning of the PTC elements during the assembly of the heat-generating element, and optionally also for holding the PTC elements during long-term operation. Because the positioning frame is made of plastic as an injection-moulded part, it consequently has certain insulating characteristics. It has been seen, however, that in generic heat-generating elements when high voltages are used, an electric flashover cannot always be avoided, due to a low resistance to leakage current.